Oskar Backlund was born in Sweden and educated at the University of Uppsala. He spent his entire career in the Russian empire, at the Dorpat (now Tartu, Estonia) Observatory, Pulkovo Observatory, and the Imperial Academy of Sciences in Petrograd. He was director of Russia’s national observatory at Pulkovo from 1895 to 1916. Most of his research was devoted to attempting to compute the difficult orbit of Comet Encke. He employed large staffs, including many women, and recomputed all planetary perturbations, publishing six volumes on the perturbations of the comet’s orbit due to the gravitational effects of Mercury, Venus, Earth, Mars, Jupiter, and Saturn from 1819 to 1891. He used the comet to estimate the masses of Mercury and Venus. He eventually concluded that the comet’s motion was affected by nongravitational forces, such as resistance by an interplanetary medium and an unknown effect that coincided with sunspot maxima. He also measured variations in latitude caused by the wobble of the earth’s axis. Note: Two of his great grandsons have persuaded me to give his first name as Johan, but some documents, including the Dictionary of Scientific Biography, list it as Jöns. In Russia his name was written Oskar Andreevich Baklund. All agree that he was known as Oskar.